Basic Manual Water Level Measurement

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Information about Basic Manual Water Level Measurement
Technology

Published on March 20, 2014

Author: HydroGResources

Source: slideshare.net

Description

This course is a self-directed course designed to introduce beginners to the concept of taking manual water level measurements in groundwater wells.

MANUAL GROUNDWATER LEVEL MEASUREMENT BASICS 2014 ©HydroG Resources Group Inc.

Accurately measuring the water level in a well is a fundamental skill for hydrogeologists and technicians. This course introduces some basic concepts and methods for beginners.

• Introduce basic concepts1 • Review standard protocols2 • Illustrate practical field methods3 Course Outline

Knowledge Base We are assuming you are familiar with : • common terms such as elevation; groundwater; aquifer; unconfined; water table; recharge; groundwater flow; etc. • concepts of quality control and quality assurance. For simplicity all our examples deal with the water table in an clean unconfined aquifer.

BASIC CONCEPTS

Why Measure Water Level? 1) Site Characterization – direction of flow • Groundwater flows from areas of higher water level elevation to areas of lower water level elevation. • Measuring the depth to water at separate observation points, and converting those measurements to elevations, allows us to determine groundwater flow directions. • Observation points can include monitoring wells or staff gauges (for surface water), etc.

aquifer well top of casing 116.5 masl water level 10.3 mbtoc water level elevation 106.2 masl well WL = 107.7 masl well WL = 105.0 masl well WL = 100.0 masl well WL = 100.9 masl well WL = 97.7 masl well WL = 95.8 masl well WL = 93.6 masl river staff gauge WL = 93.1 masl

Why Measure Water Level? 2) Site Characterization – groundwater level • Construction and development planning often requires knowledge of groundwater levels within a specific area. • Can include both depth to water (below ground surface) and groundwater elevation. • Affects excavation depth and controls, dewatering needs, foundation design, etc.

Why Measure Water Level? 3) Aquifer Characterization and Utilization • Optimal design and use of pumping wells for water supply (or other uses) requires static and pumping water level measurements. • Allows you to examine connections and responses within flow systems. • Pumping tests, slug (response) tests, routine (compliance) monitoring, etc., all require accurate measurements.

In The Field - What You See At Surface Typical Water Supply Well • large diameter (>4”) steel casing and removable lid. • submersible pump installed using pitless adapter. Side View From Above

In The Field - What You See At Surface Typical Monitoring Well • small diameter (e.g. 2”) PVC well, screened at depth. • protective (locking) casing installed at surface. Well Cap Removed Unlocked and Open Protective Casing Locked

Well Construction – Below Surface • Essentially a tube or straw installed into the aquifer. • Solid upper casing and perforated screen at bottom. • Annular seals, casing material, etc. according to local regulations. Well Screen Well casing Top of well (measuring point) Aquifer (sand) Static Level

How A Well Works • Water flows freely in and out of the well through well screen. • Air moves freely in and out of well casing. • Water level in the well reflects conditions in aquifer at the depth of the screened interval at that time. water flow groundwater flow recharge airflow Aquifer (sand)

Why Water Levels Change • Water levels change over time. • Natural seasonal changes or annual (long-term) changes. • Induced short term changes, for example due to pumping. static pumping change due to water taking seasonal fluctuation

What Is Measured • The distance (D) between an established reference point (e.g. top of well casing) and the water level in the well. • If the reference point elevation is known you can subtract the measurement (D) to obtain the water level elevation. Top of well (measuring point) Static Level Distance(D)

What This Means To You Wells provide us measuring points within the groundwater (e.g. aquifer) system. Measuring water levels allow us to assess water level and “head” (water level elevation) conditions at the well screen location. Hydrogeologic assessments rely on this data. “Garbage in = garbage out” – you need accurate measurements for accurate assessments.

STANDARD METHODS

Standard Method SOP’s Standard Operating Procedures developed to ensure highest level of accuracy. These “classic” methods are difficult to implement in field. For Example: • USGS. • ISO 2005.

United States Geological Survey Reference: Introduction to Field Methods for Hydrologic and Environmental Studies, Open-File Report 01-50 (see USGS website). Accuracy of ±0.01 feet (0.3 cm). Summary: • Cover lower portion of steel measuring (e.g. survey) tape with carpenters chalk. • Lower weighted tape into water to convenient depth, record as D1 • Raise to surface, record wetted length as D2. • Depth to water = D1-D2

ISO 2005 Reference: International Standard ISO 21413, Manual Methods for the Measurement of a Groundwater Level in a Well, (see ISO). Summary: • Similar to USGS method. • Listed accuracy of ±1 cm to depth of 60 m, and ±2 cm from 60 to 150 m. • Recommends repeat measurement and calibrating field tapes against “standard” tape.

Challenges • Check out this link for further illustration: http://www.kgs.ku.edu/Hydro/Levels/OFR96_39/ • Need to chalk tape for each measurement. • Record multiple measurements (D1 and D2). • Calculate each depth to water. • Difficulty with rapidly changing water level, condensation or “cascading” water. • For highest accuracy may need to correct for thermal expansion and stretch.

Other Manual Methods • “Bubbler” with tube in water, relies on back pressure to reflect water depth – generally lower accuracy and difficult to maintain. • Popper on tape or line, makes sound when it hits the water – lower accuracy. • Floats, often with chart recorder – moderate accuracy, higher maintenance. • Acoustic probe – moderate accuracy. • Electric Water Level Tape – commonly used, moderate to good accuracy.

SOP for Electric Water Level Tape Reference: Groundwater Technical Procedures of the US Geological Survey: Measuring water levels by use of an electric tape (GWPD4, 2010). • Detailed outline of the procedure • Excellent reference for your organization

PRACTICAL FIELD METHODS

This summary illustrates one commonly used method to measure water levels in the field. Your organization may have it’s own SOP so consult other information sources as needed. Like everything – the care you take in your measurements will reflect the accuracy of your data. There is no substitute for common sense, so adapt to your own situation, and good luck!

Some QA/QC Tips  Sometimes you need to determine if small changes in water level occur over time.  Variations in methods or equipment used can result in significant measurement differences (that are not related to water level).  It becomes difficult to determine if measurement differences between monitoring events are due to water level changes or QA/QC problems.  STANDARD METHODS ARE IMPORTANT.

Some QA/QC Tips – Reference Point • Establish and mark a reference (measuring) point on each well. • Ensure everyone taking measurements uses the same reference point to measure from. • Try not to change the reference point. Here is one example. (you can likely create a better method/mark)

Equipment • Commercially available electronic water level meters are commonly used. Two Examples • Flat line tape for most applications. • Thin-line (coaxial cable) tape for narrow diameter wells or access holes.

Equipment Use • Meters are essentially measuring tapes with a weighted probe at the bottom end. • The probe has two electrode ends connected to a simple battery powered circuit. • The probe is lowered into the well, the reel is held at surface. • When the probe enters the water the circuit is connected and an indicator light and/or sound is activated at the reel. • Often you can adjust the sensitivity, etc., for varying conditions.

Equipment Use • When the indicator sounds, read the depth to water on the measuring tape from the reference point. In this example the water level measurement is recorded as: 1.02 metres below top of well

Some QA/QC Tips – Measuring • Follow the manufacturer's instructions. • Lower the probe slowly into the well. • Stop when the indicator sounds continuously. • Move the probe/tape up and down slightly (out of then back into the water) a few times until you are certain you have the correct measurement. • Accuracy of ±1 cm is considered typical.

Some QA/QC Tips – Recording Data • Record as much detail as possible: field personnel, site location, date Well identifier, time of measurement measuring point used water level meter used conditions of well weather conditions etc., etc., etc.

Common Issues • Tapes can stretch (or be damaged). Number and compare the meters you are using at a site to determine relative accuracy or variation. • Beware of “false positives” – water on inside of the well casing or cascading water can create false positive readings. You should get the same measurement repeatedly when you raise and lower the probe. • Low (or high) conductivity water, or presence of hydrocarabons (for example), can lead to false readings.

When you are trying to do 10 things at once……. Some consistency helps!

WE HOPE YOU ENJOYED THIS COURSE – PLEASE ALSO SEE OUR DATALOGGER COURSES 2014 ©HydroG Resources Group Inc.

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